The invention relates to a process for improving the adhesion of radiation-curable acrylate resins comprising amino-functional acrylates on substrates by admixing and amine-hardening a polyepoxide compound.
Radiation-curable coating materials based on acrylate resins are known and are often used for coatings on substrates such as metals or polymer moldings. The polymerization shrinkage which occurs in the course of radiation curing of the coating film applied to the substrate has an adverse effect on the adhesion of the coating material to said substrate. It is known that the adhesion of the radiation-curable coating film may be improved by increasing the molecular weight of the film-forming resins, reducing their double bond density, or adding non-reactive polymers. This, however, leads to a sharp and unwanted increase in the viscosity of the coating materials. Without doubt there exists a need for radiation-curable coating materials which are based on acrylate resins and possess good adhesion to metal or plastics substrates.
We have found that the adhesion of radiation-curable coating materials based on acrylate resins as binders to substrates is improved if the coating materials comprise at least one compound having amine groups and unsaturated acrylate groups and also comprise a small amount of polyepoxide compounds and if the coating film applied to the substrate is cured with high-energy radiation and stored, or thermally conditioned, at a temperature of above 50xc2x0 C., in particular of from about 50 to 120xc2x0 C.
The present invention therefore provides a process for improving the adhesive strength of radiation-curable acrylate resins or mixtures thereof, comprising a compound containing at least one amine group and at least one radiation-curable unsaturated acrylate group, to substrates, which comprises admixing the acrylate resins with at least one polyepoxide compound having an epoxide value of from 1 to 15 mol/kg, curing the film of the resultant acrylate resin, applied to the substrate, using high-energy radiation, and conducting at least partial amine hardening of the polyepoxide compounds by treatment at a temperature of above 50xc2x0 C.
The acrylate resin mixture obtained by admixing the polyepoxide compound(s) is sometimes referred to below as the coating material or coating mixture.
It was surprising that the process not only led to improved adhesive strength between the substrate such as a metal sheet or polymer film and the cured coating film but also brought about a reduction in cracking in the coating films and a reduction in the yellowing of the coating materials. A further surprise was the good storage stability of the uncured mixtures in comparison to mixtures which comprise epoxy resins and low molecular mass aliphatic amines as hardeners.
Acrylate resins are understood to be known reaction products, in resin form, of (i) methacrylic acid and/or acrylic acid with (ii) at least dihydroxy polyesters, polyethers, polyurethanes or epoxy resins (which comprise at least two functional groups which react with (meth)acrylic acid), and also reaction products of (i) hydroxyalkyl (meth)acrylates with (ii) compounds containing isocyanate groups. Radiation-curable acrylate resins of this kind are customary in commerce and are described, for example, in P. K. T. Oldring, Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints, Vol. II: Prepolymers and Reactive Diluents, J. Wiley and Sons, New York and Sita Technology Ltd., London 1997, and in H. Kittel, Lehrbuch der Lacke und Beschichtungen [Textbook of Paints and Coatings], Volume VII: Processing of Paints and Coating Materials, pp. 240-245 etc., Verlag W. A. Colomb, Berlin 1979. The acrylate resins, which are subdivided, in accordance with their preparation and the repeating structural units in the molecule chain, into polyester acrylates, polyether acrylates, urethane acrylates, epoxy acrylates and melamine acrylates, are, as what are known as radiation-curable prepolymers, of relatively low molecular mass, generally having an average molecular weight Mn of from 300 to 15,000 and preferably from 400 to 3000 g/mol, as determined by gel permeation chromatography (GPC) using polystyrene as the standard and tetrahydrofuran as the eluent. The resins contain generally from 0.1 to 1.0 and preferably from 0.1 to 0.5 mol of polymerizable Cxe2x80x94C double bonds per 100 g of prepolymer. Very suitable (meth)acrylate resins contain from 2 to 20, in particular from 2 to 10, and preferably from 2 to 6, methacryloyl and/or acryloyl groups in the molecule. Among the acrylate resins, particular suitability is possessed by those derived from polyfunctional aliphatic alcohols having no functional groups other than the hydroxyl groups, except for ether, ester, and urethane groups. Examples of alcohols are dihydric, trihydric and higher polyhydric alcohols such as propylene glycol, diethylene glycol, triethylene glycol, butanediol, hexanediol, neopentyl glycol, cyclohexanediol, glycerol, trimethylolpropane, ditrimethylolpropane, pentaerythritol, dipentaerythritol, and sorbitol. Compounds suitable for preparing polyester acrylates are primarily aliphatic polyester polyols. Polyester acrylates may be prepared in one or more stages from polyols, polycarboxylic acids, and (meth)acrylic acid. They are described, for example, in EP-A 279303. Alcohols suitable for preparing polyether acrylates are, in particular, alkoxylated, preferably ethoxylated and/or propoxylated, polyhydric alcohols, in which the degree of alkoxylation per hydroxyl group may be from 0 to 10.
In accordance with the process of the invention, the acrylate resins in the mixture comprise compounds having at least one amine group and at least one radiation-curable unsaturated acrylate group, especially amine-modified acrylate resins having a molecular weight Mn of at least 300 and preferably at least 400 g/mol. The mixtures may be of amino-free acrylate resins with compounds having at least one amine group and at least one radiation-curable unsaturated acrylate group, such as an amine-modified acrylate resin, although exclusively amine-modified acrylate resins may also be used with advantage as binders. By amine-modified acrylate resins are meant here acrylate resins which comprise Michael adducts of aliphatic amines with primary and/or secondary amino groups. These may be amine-modified polyether, polyester, epoxy and urethane acrylates, with polyether and polyester acrylates being preferred. Highly suitable acrylate resins are those in which from 0.5 to 60, and in particular from 0.5 to 30, mol % of the (methlacrylic groups are present in the form of Michael adducts of an amine having a primary and/or secondary amino group. The preparation of amine-modified acrylate resins is described, for example, in Patent Applications DE-A 2346 424, DE-A 4007 146, EP-A 211 978, EP-A 280 222 and EP-A 731 121. Amine-modified acrylate resins particularly suitable for the process of the invention are those which have an amine number of from 5 to 450 and preferably from 20 to 250 mg KOH/g. The mixture of the coating components should contain an amine number of from 5 to 250, in particular from 5 to 100, and preferably from 20 to 50 mg KOH/g. Amine synergists based on multifunctional monomers may also be used in some cases.
In addition to the compound having amine groups, the coating mixtures of the invention comprise at least one amine-hardenable polyepoxide compound having an epoxide value of 1-15 and especially 3-8 mol/kg. Preference is given to aliphatic polyepoxides and also aliphatic or aromatic glycidyl ethers and glycidyl esters having at least 2 glycidyl groups. Highly suitable polyepoxides are aliphatic glycidyl ethers such as pentaerythritol triglycidyl ether, trimethylolpropane triglycidyl ether, glycerol triglycidyl ether, and neopentyl glycol diglycidyl ether. The epoxide groups of the polyepoxide compounds may also have been partially reacted with (meth)acrylic acid, subject to the proviso that two intact epoxide groups remain in the molecule. The molecular weights of suitable polyepoxide compounds are generally below 2000 and preferably below 1100 g/mol. The amount of the epoxide compounds in the mixture is guided by the epoxide value thereof and by the amount of the other constituents in the mixture; the acrylate resin mixture should have an epoxide value of from 0.1 to 4, in particular from 0.1 to 2, and preferably from 0.1 to 1, mol/kg.
For the purpose in particular of adjusting the viscosity or influencing the hardness of the coating films, the acrylate resin mixtures used in accordance with the invention may further comprise what are known as reactive diluents, i.e., radiation-curable monomers having in particular 1-4 Cxe2x80x94C double bonds. Appropriate monomers and the criteria for their selection are described, for example, in the book cited above in connection with the radiation-curable acrylate resins, by P. K. T. Oldring, Vol. II, Chapter III, pp. 261-325. Customary additives, furthermore, may also be added to the acrylate resin mixtures, such as matting agents, fillers, pigments, leveling assistants, etc. The amount of these customary additives is generally from 0.01 to 20 and preferably from 0.05 to about 10% by weight of the total amount of the mixture.
The acrylate resin mixtures may be applied conventionally to substrates such as metals or plastics, examples being polyethylene, polypropylene, polyurethanes etc., such as by knife coating, spraying, flow coating, or rolling. Layers of the acrylate resin mixtures can be applied to a substrate.
The radiation curing of the coating films can be carried out with the aid of high-energy radiation, such as UV rays, electron beams, or gamma rays. Preference is given to curing with UV light. In this case it is necessary to admix to the mixtures at least one photoinitiator in an amount of from 0.05 to 20 and preferably from 0.05 to 5% by weight, based on the total amount of the acrylate resin mixture. In this respect, reference may be made to the extensive literature, for example, to P. K. T. Oldring, Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints, SITA Technology, London 1991, Vol. III: Photoinitiators for Free Radical and Cationic Polymerisation. Examples of photoinitiators that may be mentioned include benzophenone, alkylbenzophenones, halogenated benzophenones, Michler""s ketone, anthrone, anthraquinone and its derivatives, benzoin and its derivatives, and also acylphosphine oxides such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide. The reactivity in UV polymerization may be increased in a conventional manner by adding tertiary amines such as triethylamine, triethanolamine, or amine synergists (e.g., Laromer(copyright) LR 8956). Radiation curing with UV light is suitably conducted using artificial emitters whose emission lies within the range of 2500-5000, preferably 2500-4000, angstroms. Suitable emitters are mercury vapor lamps, xenon lamps and tungsten lamps, with preference being given to the use of high-pressure mercury emitters. Radiation curing gives a scratch-resistant coating film.
For thermal curing or partial curing of the polyepoxide-amine system, the irradiated coating film is treated (thermally conditioned) at temperatures of above 50xc2x0 C., in particular from 50 to 120xc2x0 C., and preferably from 60 to 120xc2x0 C. The temperature and duration of thermal conditioning are codetermined by the specific epoxy/amine system used and may easily be optimized in preliminary experiments. In general, thermal conditioning is carried out for from about 5 to 240 minutes, often longer. It has been found that the curing reaction initiated by the thermal conditioning often continues for a relatively long time, even if the coating has been cooled to room temperature in the meantime, thereby leading to a further improvement in the adhesive strength.
The resultant acrylate resin films have the surprising, advantageous properties indicated above. They are therefore particularly suitable for producing coatings on metal and plastic.
The examples and comparative experiments below are intended to illustrate, but not restrict, the invention.
Unless specified otherwise, all parts and percentages are by weight.
The epoxide value, or the epoxide equivalent weight, was determined in accordance with DIN 53188 by titrating a solution of resin in a dichloromethane-acetic acid mixture, with the addition of tetra-n-butylammoniumiodide and crystal violet as indicator, with 0.1 N perchloric acid up to the point of color change from blue to yellowish green. The epoxide value indicates the number of moles of epoxide groups present in 100 g of an epoxy resin. The following relation exists: epoxide value=100/epoxide equivalent weight.
The amine number indicates the number of mg of KOH that are equivalent to 1 g of the substance. It is determined by dissolving 1-2 g of the epoxy resin sample in 50 ml of acetic acid, adding crystal violet solution as indicator, and titrating with 0.1 N perchloric acid in acetic acid until the point of color change of the indicator from blue to yellowish green.
As a measure of the flexibility, the measurement was made in accordance with DIN 56156 of the Erichsen indentation (EI), in mm, both before and during thermal conditioning, with high values denoting high flexibility.
In order to test the adhesion, the cross-cut value (GT) was determined in accordance with ISO 2409 (DIN 53151), both with and without Tesa film tearoff (mT), and before and during thermal conditioning.
Additionally, in Table 3, the adhesive strength in N/mm2 was measured using the Twistometer torsion measuring instrument on a coating applied at 25 g/m2 to a deep-drawn metal panel. The method is described in Farbe und Lack, Vol. 80, 10/1974 and also in the BASF datasheet xe2x80x9cBestimmung der Haftfestigkeitxe2x80x9d [Determination of Adhesive Strength] PM/ED 038d (Jan. 1999). By continuous rotation of a shaft, the Twistometer raises the torque on pre-prepared adhesive plugs up to the fracture load; when this load is reached, a pointer remains fixed in position and permits the adhesive strength to be read off directly in N/mm2 on a scale on the instrument.
The yellowing was determined using a Lange Colorpen spectrophotometer (400-700 nm; 20 n) in analogy to DIN 6167, ISO 7724/1-3 and ASTM D 1925-70, and is reported as the b value.
The viscosity (mPa.s) was determined in accordance with DIN EN ISO 3217.
As a measure of the reactivity, the belt speed is reported at which coating film applications of 50 g/m2 can be moved under an undoped high-pressure mercury lamp (output 120 W/cm lamp length; distance of lamp from substrate: 12 cm) in order to give a coating which resists scratching with the fingernail.